unused_mut wrongly warning when using a macro

[jeromegn]

Reproduction:

use tokio::select;
use futures::future;

#[tokio::main]
async fn main() {
    let v = async move {
        let ok = future::ready(1);
        tokio::pin!(ok);
        select! {
            mut a = &mut ok => {
                a += 1;
                a
            }
        }
    }.await;
    println!("v: {}", v);
}

I expected to see this happen: no warnings.

Instead, this happened:

warning: variable does not need to be mutable
  --> src/main.rs:10:13
   |
10 |             mut a = &mut ok => {
   |             ----^
   |             |
   |             help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default

warning: 1 warning emitted

If I follow the advice here, it fails to compile with:

error[E0384]: cannot assign twice to immutable variable `a`
  --> src/main.rs:11:17
   |
10 |             a = &mut ok => {
   |             -
   |             |
   |             first assignment to `a`
   |             help: make this binding mutable: `mut a`
11 |                 a += 1;
   |                 ^^^^^^ cannot assign twice to immutable variable

error: aborting due to previous error

This is possibly related to #60643, but I'm not sure. This one is not in a macro itself, it's using a macro.

Meta

rustc --version --verbose:

rustc 1.45.0 (5c1f21c3b 2020-07-13)
binary: rustc
commit-hash: 5c1f21c3b82297671ad3ae1e8c942d2ca92e84f2
commit-date: 2020-07-13
host: x86_64-unknown-linux-gnu
release: 1.45.0
LLVM version: 10.0

(but this has been happening on every version I've tried for over a year)

[SNCPlay42]

Looking at the source, tokio::select expands mut a (as $bind) at two different locations and the mutability is indeed unnesscessary at one of them:

match &out {
    $bind => {}
    _ => continue,
}

I'd consider this a tokio bug. There should be a #[allow(unused_mut)] on the match (like there already is a #[allow(unused_variables)]).

[Dylan-DPC]

This code doesn't emit the error any more